The invention relates to a device and a process for monitoring the capacity of a battery, in particular in key transmitters or remote controls such as in automotive applications, comprising measuring means for the terminal voltage and means for calculating the internal resistance of the battery as well as further means to determine the state of charge of the battery from the measured values.
A device of the type mentioned in the introduction is known from DE-OS 44 18 194. It is a system for determining the capacity of a chargeable storage battery of an automobile, in particular for determining the residual capacity (state of charge) of such a battery. From this printed publication it is known that the residual battery capacity is a function of electrolyte density, and that the electrolyte density can be correlated to the internal resistance of the battery and the discharge current of the battery. In this, the internal resistance of the battery is calculated by acquiring the terminal voltage in an uninterrupted state in which the connections to all electricity-consuming devices are interrupted, and a terminal voltage different from this in a selectively-connected state in which a connection is established to only one electricity-consuming device, as well as from the discharge current in this connected state. Acquisition of the residual capacity of the battery subsequently takes place at the temperature condition valid at the time, in connection with a table which shows the context between battery residual capacity, discharge current and internal resistance.
This process is expensive, because calculations in several steps must be carried out to do this, and in addition because switching off the battery from all electricity-consuming devices or selectively from individual electricity-consuming devices requires considerable switching action.
Apart from this, from DE-PS 34 07 409 a testing process for batteries is known in which the internal resistance of the battery is measured and from the comparison with a direct-current source of the same type as the device under test, of a determined state of charge, the state of charge of the device under test is determined. This process is expensive, because a second reference battery must always be at hand.
Finally, from DE-OS-38 18 034 a measuring device is known which directly indicates the internal resistance of batteries. Measurement is via a frequency-band-limited amplifier stage by pulsing of the battery as a test object with an alternating-current source.
On the other hand, from practical applications, processes are known which carry out measurements of a battery by monitoring the terminal voltage at the battery, because the voltage of a typical lithium cell at a constant load towards the end of its life progressively decreases. If however, a lithium cell is only occasionally subjected to a load, as is for example the case with a key transmitter, then the cell voltage always approaches the equilibrium value. In such a case, a method for estimating battery capacity which only refers to battery voltage is not suitable because, as a result of the temporary "recovery" of the battery, a fully-charged battery is simulated.
A further disadvantage of pure voltage measurement results from the strong temperature dependability of the voltage progression which is described by the Nernst equation: EQU .phi.=.phi..sub.0 +(R*T/z*F)*1n(MWG)
Here, the electrode potential .phi. is given by the respective standard electrode potential .phi..sub.0 and a further term into which the absolute temperature enters in a linear way. The voltage collapse observed towards the end of the service life thus also depends on the temperature and can only partially be used for assessing the capacity.
It is the object of the invention to develop a device or a process which make it possible to provide reliable information concerning the available cell capacity (state of charge), whereby in particular the capacity of a cell, even in cases of only occasional exposure to loads, is to be determined correctly. In addition, the measuring result should be as far as possible independent of temperature.